1. Field of the Invention
The present invention generally relates to three-port nonreciprocal circuit devices, and more particularly, relates to a three-port nonreciprocal circuit device, such as an isolator or circulator, used in a microwave band, and also relates to a communication apparatus including the nonreciprocal circuit device.
2. Description of the Related Art
Typically, isolators operate so as to allow signals to pass only in the transmission direction and to block the transmission in the opposite direction, and are used in transmission circuit sections of mobile communication apparatuses, such as car phones and portable telephones.
Conventionally, as isolators of this type, three-port isolators (isolators having three, i.e., first to third, center electrodes) have been known. As shown in FIG. 12, an isolator 100 includes center electrodes 101, 102, and 103, a ferrite element 110, matching capacitors 105, 106, and 107, and a terminating resistor 108. A port portion P1 is connected to one end of the center electrode 101. An input terminal 114 and the matching capacitor 105 are electrically connected to the port portion P1. A port portion P2 is connected to one end of the center electrode 102. An output terminal 115 and the matching capacitor 106 are electrically connected to the port portion P2. A port portion P3 is connected to one end of the center electrode 103. The matching capacitor 107 and the terminating resistor 108 are electrically connected to the port portion P3. The matching capacitors 105, 106, and 107 and the terminating resistor 108 are connected to corresponding ground.
Meanwhile, in typical communication apparatuses, amplifiers used in the circuits thereof cause signals to be distorted to some extent. This distortion causes spurious components, such as a second harmonic (2 f) and a third harmonic (3 f) of an operating frequency f, to be generated, which is responsible for unwanted emissions. Since unwanted emissions in communication apparatuses causes malfunction and/or interference of power amplifiers, standards and specifications are specified in advance. In order to prevent unwanted emissions, a method in which a filter or the like is provided is commonly used to attenuate unwanted frequency components. The use of such a filter, however, leads to a problem in that loss occurs because of the filter, which is undesirable.
Accordingly, a possible approach for suppressing spurious components is to utilize characteristics of bandpass filters included in the isolators or circulators. However, the nonreciprocal circuit device having the basic conventional configuration shown in FIG. 12 cannot provide sufficient attenuation characteristics in an unwanted frequency band.
To overcome the problem, Japanese Unexamined Patent Application Publication Nos. 2001-320205 and 2001-320206 disclose nonreciprocal circuit devices that can provide large attenuation in, mainly, a frequency band in which spurious components, such as a second harmonic (2 f) and a third harmonic (3 f) of an operating frequency (f), are generated. FIG. 13 is an equivalent circuit diagram of an isolator that is one example of the nonreciprocal circuit devices of the related art.
This isolator 120 is different from the isolator 100 shown in FIG. 12 in that a series inductor 121 is electrically connected between the matching capacitor 106 and ground. Thus, the matching capacitor 106 and the series inductor 121 constitute a trap circuit, which makes it possible to attenuate signals in a frequency band away from the passband.
FIG. 14 is a graph showing the attenuation characteristics of the isolator 100 (Conventional Example 1) shown in FIG. 12 and the isolator 120 (Conventional Example 2) shown in FIG. 13. Both of the isolators 100 and 120 have a bandpass of 900 MHz. From FIG. 14, it can be seen that Conventional Example 2 displays increased attenuations of a second harmonic (2 f) and a third harmonic (3 f) compared to Conventional Example 1.
As discussed in Japanese Unexamined Patent Application Publication No. 2001-320205, one end of each of the three center electrodes 101, 102, and 103 in the isolator 120 is electrically connected to a common ground portion having the same shape as the bottom surface of the ferrite element 110. This common ground portion is brought into contact with the bottom surface of the ferrite element 110. The three center electrodes 101, 102, and 103 extending from the common ground portion are bent so as to be spaced 120 degrees with respect to one another and are arranged on the upper surface of the ferrite element 110 with an insulating sheet interposed therebetween.
However, while the isolator 120 having the trap circuit, which is constituted by the matching circuit 106 and the series inductor 121, as shown in FIG. 13, can increase the attenuations of the second harmonic (2 f) and the third harmonic (3 f) of the operating frequency of a communication apparatus, there are problems in that the insertion loss and return loss characteristics deteriorate and the band width ratio decreases.
FIG. 15 is a graph showing the insertion loss characteristics of the isolator 100 (conventional example 1) shown in FIG. 12 and the isolator 120 (conventional example 2) shown in FIG. 13, and FIG. 16 is a graph showing the output return-loss characteristics thereof. From FIGS. 15 and 16, it can be seen that the band width ratio of the isolator 120 decreases.
In order to overcome the problems described above, preferred embodiments of the present invention provide a three-port nonreciprocal circuit device and a communication apparatus which prevent the propagation of a second harmonic (2 f) and a third harmonic (3 f) of an operating frequency f without deterioration of the insertion loss and return loss characteristics.
A three-port nonreciprocal circuit device according to a preferred embodiment of the present invention includes:
(a) a ferrite element;
(b) a permanent magnet for applying a direct-current magnetic field to the ferrite element;
(c) a first center electrode arranged at a major surface of the ferrite element or in the ferrite element, one end of the first center electrode being electrically connected to a first port;
(d) a second center electrode arranged at the major surface of the ferrite element or in the ferrite element so as to cross the first center electrode in an electrically insulating state, one end of the second center electrode being electrically connected to a second port;
(e) a third center electrode arranged at the major surface of the ferrite element or in the ferrite element so as to cross the first center electrode and the second center electrode in an electrically insulating state, one end of the third center electrode being electrically connected to a third port;
(f) at least one matching capacitor constituting an LC parallel resonator circuit in conjunction with one of the first, second, and third center electrodes, and
(g) at least one series inductor electrically connected between one of the at least one LC parallel resonator and ground.
In addition, the three-port nonreciprocal circuit device is preferably constructed such that the other end of at least one of the first, second, and third center electrodes is not connected to a common potential and does share a common end with another end.
With the arrangement described above, a circuit in which each LC parallel resonator circuit, constituted by the center electrode and the matching capacitor, and the corresponding series inductor are connected provides a trap circuit. This trap circuit can increase the attenuations of the second harmonic (2 f) and the third harmonic (3 f) of the operating frequency f of a communication apparatus without deterioration of the insertion loss and return loss characteristics. The resonant frequency (trap frequency) of the trap circuit, constituted by the LC parallel resonator circuit and the series inductor, is preferably in the range of about 1.5 to about 3.5 times the operating frequency.
The inductances of the series inductors, each electrically connected between the corresponding LC parallel resonator circuit and ground, may be different from each other. In this case, the trap frequencies of the trap circuits can be made different from each other. Thus, for example, setting the trap frequency of one trap circuit to be in the vicinity of the second harmonic (2 f) and setting the trap frequency of another trap circuit to be in the vicinity of the third harmonic (3 f) can further increase the attention of both the second harmonic (2 f) and the third harmonic (3 f).
The at least one matching capacitor preferably includes a capacitor electrode and the at least one series inductor includes an inductor electrode. The capacitor electrode and the inductor electrode may be provided in a multilayer substrate in which insulating layers are stacked. This can reduce the number of connections soldered between the matching capacitor and the series inductor and can increase connection reliability.
A communication apparatus according to-another preferred embodiment of the present invention includes the three-port nonreciprocal circuit device described above. This communication apparatus, therefore, can improve frequency characteristics.
Accordingly, the present invention can provide a three-port nonreciprocal circuit element and a communication apparatus which are improved in performance and reliability and are reduced in size.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached figures.